Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers and are subsequently converted to ceramic materials, in particular, silicon carbide/silicon nitride bearing fibers by a two-step process of curing to render the preceramic polymeric fibers insoluble followed by pyrolyzation comprising heating the fiber in an inert atmosphere up to about 2,000.degree. C. whereupon the fibers are converted to ceramic form.
U.S. Pat. No. 3,853,567 is an early example of thermally treating a polysilazane resin to form ceramic articles comprising silicon carbide and/or silicon nitride. Thus, in Example 1 of the patent, a carbosilazane resin is formed, spun into filaments, the filaments rendered infusible by treating them with moist air for 20 hours at 110.degree. C. and subsequently heated over the course of 7 hours to 1,200.degree. C. in a nitrogen atmosphere and then to 1,500.degree. C. over the course of 5 minutes. A black-glistening filament which is completely insensitive to oxidation at 1,200.degree. C. and is amorphous to X-rays is disclosed as obtained. Subsequent heating to 1,800.degree. C. under argon produced a fiber consisting of .beta.-SiC, a little .alpha.-SiC and .beta.-SiC.sub.3 N.sub.4.
While silicon carbide- and silicon nitride-based ceramics have potential use in high-temperature applications as substitutes for metal super-alloys, it has recently been found that an alloy of SiC and AlN in comparison with SiC possesses superior creep resistance, improved fracture toughness, lower thermal conductivity, and possibly enhanced oxidation and corrosion resistance. Thus, Rafaniello et al have reported that by using AlCl.sub.3.6H.sub.2 O, starch and SiO.sub.2 fine powder as starting materials, and heating, a sintered powder comprising a SiC-AlN solid solution having improved properties relative to SiC was prepared (Journal of Materials Science 16 (1981) 3479-3488). Shaped articles were prepared by hot pressing the powder.
Sintered ceramic articles prepared by hot pressing ceramic powders are restricted to simple configurations and are incapable of being formed into more complex shapes. Accordingly, hot pressed ceramics are not yet totally satisfactory as useful engineering ceramics. Alternatively, a pressureless sintering method has been developed to form ceramic articles based on SiC, in general, as well as from SiC-AlN solid solutions. In the pressureless sintering method, a suitable sintering aid is incorporated, whereby a green body of silicon carbide powder is sintered in an atmosphere under atmospheric pressure or under pressure in the vicinity of atmospheric pressure. By this method, it is possible to obtain a highly dense high strength sintered article having any desired shape. However, the strength, particularly the high temperature strength of ceramics formed by this pressureless sintering method is still inadequate and has processing drawbacks. A good summary of the current art of forming silicon carbide, silicon carbide/silicon nitride, and silicon carbide/aluminum nitride articles from inorganic powder is set forth in U.S. Pat. No. 4,569,922 which is herein incorporated.
While processes of forming ceramic articles from inorganic powders has been found incapable of satisfactorily producing ceramic articles of complex shapes, the formation of ceramic articles from preceramic organometallic polymers is starting to overcome the processing difficulties found using inorganic starting materials. Not only have silicon carbide and silicon nitride articles, including fibers, been formed from preceramic organosilicon polymers as discussed above, alumina fibers have been formed from organoaluminum compounds including organoaluminum polymers. Thus, U.S. Pat. Nos. 4,514,555 and 4,533,712 disclose processes of producing high molecular weight organoaluminum polymers comprising reacting water and an organoaluminum compound represented by the formula: ##STR1## wherein X, Y and Z are each a hydrogen atom, an alkyl group, and alkoxyl group, or an acyloxyl group and an organic acid. The polymers can be spun into fibers and then heated to produce alumina fibers.
Ceramic articles from organometallic polymers are further disclosed in U.S. Pat. No. 4,097,294. This patent discloses selecting one or a mixture of polymers selected from the class consisting of poly(diorganosilanes), poly(haloorganosilanes), poly(carbosilanes), polysilazanes, polycarbocarboranes and polyborazines and heating the polymers to produce the ceramic. This patent also discloses a mixed ternary ceramic obtained from a polymer containing repeating units of a framework comprising carbon, boron, silicon and siloxane components.
U.S. Pat. No. 4,105,455 discloses a method of producing a sintered silicon carbide body by the steps of forming a polycarbosilane which is insoluble in solvents and unmeltable, pulverizing the insoluble and unmeltable carbosilane so as to form a powder and applying heat with or without the use of pressure to the powder to decompose the powder into silicon carbide. In Example 8 of this patent there is disclosed a process of forming a polycarbosilane soluble in n-hexane, mixing therewith aluminum-iso-propoxide and heating to obtain an insoluble and unmeltable polycarbosilane containing a small amount of aluminum. The mixture is pulverized and charged into a graphite mold to form a ceramic. The patent teaches adding other organometallic compounds including alkylborates and the like to the soluble polycarbosilane, heating to form an insoluble polymer and pulverizing the powder.
U.S. Pat. No. 4,172,108 discloses a process for producing sialons, i.e. solid solutions of silicon nitride and alumina. The process comprises mixing a silicon nitride precursor such as amino-or imino-silanes and an alumina precursor such as trialkoxy- or triacyloxy-aluminums or polyaluminoxanes to obtain a sialon precursor, and then heating the sialon precursor to form the ceramic.
U.S. Pat. No. 4,248,814 discloses a process for producing a heat-resistant ceramic sintered body which comprises mixing a polycarbosilane with a polyborosiloxane containing phenyl groups in at least part of the side chains of Si, heating the polymer mixture to form a polycarbosilane partly containing siloxane bonds, mixing the polycarbosilane with a ceramic powder such as metallic oxides, carbides, nitrides, borides and silicides, shaping the resulting mixture and sintering the mixture to form a ceramic.
There is still a need, therefore, to form ceramics comprising an alloy of SiC and AlN by a method which will allow fabrication of complex shapes and still retain the desired mechanical properties of ceramic materials. Up until the present time, the art has not suggested a method of forming aluminum nitride and SiC and AlN solid solutions from other than inorganic powders which have not yet been formed into satisfactory high performance ceramic articles.